Modular multilevel converter based HVDC transmission is a new generation of multilevel voltage source converter HVDC topology. Several sub modules and specific modulation are applied in a sine waveform of arm voltage. Due to the discrepancy of three-phase arm voltage, circulating current appears among the three-phase unit. The circulating current distorts the sine arm current and increases its peak value, so the requirements of current rating increase.
First the sub module consists of turn-off devices, and then the sub modules are connected in series to form a modular multilevel converter. Modular multilevel converter valve is the core and key equipment of flexible DC transmission; ft is not only the requirements for the system but also the requirements for protection itself to guarantee the converter valve safe and stable operation.